In the NTSC and PAL television systems there is a defined relationship between the horizontal line frequency (Fh) and the color subcarrier frequency (Fsc). In the NTSC color television standard, for example, Fsc equals (455/2)Fh. In the PAL standard, Fsc equals (1135/4)Fh+Fv/2 where Fv is the field frequency (50 Hz).
Some television sources provide output signals which may differ substantially from the standards noted above. Examples include consumer grade video cassette recorders, video disk players, video games and the like. It has been recognized by Nicholson et al., in U.S. Pat. No. 4,635,099 entitled APPARATUS FOR DETECTING NON-STANDARD VIDEO SIGNALS, and by Nicholson in U.S. Pat. No. 4,665,437 entitled ADAPTIVE FIELD OR FRAME STORE PROCESSOR, that such non-standard signals may degrade the operation of television receivers or monitors which employ field or frame processing of video signals for purposes such as comb filtering, progressive scan conversion or temporal noise reduction. Nicholson proposed that non-standard signals be detected so as to appropriately modify the video processing when non-standard signals are present.
An embodiment of the Nicholson et al. non-standard signal detector includes a coincidence detector which compares coincidence of line rate input signal pulses with line rate pulses derived by counting down a clock signal locked to the color burst component of the input signal and synchronized to the line rate input signal by feedback. A retriggerable monostable multivibrator, having a period equivalent to 25 line intervals, is triggered by the coincidence detector if pulse coincidence has occurred at least once in every twenty five lines. An output circuit includes a flip-flop which is set if the monostable has been triggered for at least one frame interval thereby signifying the presence of a standard video input signal. Advantageously, this latter feature of requiring positive detection of standard signals for a fixed period of time tends to reduce the "false alarm" rate of the detector. False alarms, or invalid detections, may result for video input signals having a poor signal-to-noise ratio.
Another example of a non-standard signal detector is described by Sakamoto et al. in U.S. Pat. No. 4,821,112 entitled DETECTION CIRCUIT FOR DETECTING STANDARD TELEVISION SIGNALS AND NON-STANDARD TELEVISION SIGNALS which issued Apr. 11, 1989. An embodiment of their detector includes a synchronization separation circuit, an APC circuit, a frequency division circuit connected to the APC circuit, and a comparator circuit connected to the frequency division circuit and the synchronization circuit. In operation, the frequency of the chrominance subcarrier signal is divided by the frequency division circuit and the phase of the chrominance subcarrier signal thus frequency divided is compared with the phase of the synchronizing signal separated by the synchronizing separation circuit. False alarms (detection errors) in this system are reduced by connecting an integrator to the output of the comparator circuit.